Diversity

The order Cetacea comprises two extant sub-orders and one extinct sub-order. The extant sub-orders are Mysticeti (baleen whales) and Odontoceti (toothed whales). Both mysticetes and odontocetes are thought to be descendants of archaeocetes (Archaeoceti, ancient whales), an extinct sub-order. There are at least 83 living species of cetaceans, with 46 genera in 14 families. Of the two extant suborders, Odontoceti is larger and more diverse, with at least 70 species, 40 genera, and 10 families. Cetaceans, along with bats, are considered some of the most derived mammals on the planet. They evolved from terrestrial animals to an entirely aquatic life form that is completely separated from the land in all aspects of biology. Cetaceans live, breed, rest, and carry out all of their life functions in the water. (Gingerich, et al., 2001; Reeves, et al., 2002; Rice, 1984)

Habitat

Cetaceans are exclusively aquatic. Most are species are marine, inhabiting coastal areas as well as open ocean. A few species inhabit freshwater rivers and lakes. Others live in the brackish waters of estuaries and coastal marshes. (Nowak, 1999; Rice, 1984)

Physical Description

All cetaceans share a number of similarities: they have a fusiform, or streamlined body shape; paddle-shaped front limbs; vestigial hind limbs (which are within the body wall); no external digits or claws; tail flattened laterally and bearing horizontal flukes at the tip; vestigial ear pinnae; basically hairless body (some young have hair on their snouts); thick subcutaneous blubber layer filled with fat and oil; telescoped skull bones; external nares (blowhole) on the top of the head; addition of compressed vertebrae; shortening of the neck; lack of sweat glands; internal reproductive organs; 3-chambered stomach; and an airway reinforced with cartilage to the alveoli. Many of these characteristics are adaptations to reduce drag for fast swimming in an aquatic environment. Protuberances such as external ears or genitals would create turbulence and would be inefficient for an animal in the water. (Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Cetaceans are white, black, gray, bluish-gray, or pink in color, and many are spotted, mottled, streaked, or boldly patterned. Most exhibit some countershading, tending to be lighter ventrally than dorsally. They are large animals, ranging in size from 20 to 180,000 kilograms and from 1.2 to 30 meters. Blue whales (Balaenoptera musculus) are the largest animals that have ever existed. Some species are sexually dimorphic in size. For example, female blue whales are larger than males and male bottlenose dolphins (Tursiops truncatus) are larger than female bottlenose dolphins. In a few species, such as narwhals (Monodon monoceros) and beaked whales (Ziphiidae), males have enlarged, protruding teeth that may be used in aggressive male-male encounters. (Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Some cetaceans are thought to be the most intelligent non-primates and many have proportionately large brains. They also have remarkably efficient lungs and circulatory systems, allowing them to dive for extended periods of time. Cetaceans use about 12% of the oxygen that they inhale, compared to 4% used by terrestrial mammals. They also have at least twice as many erythrocytes and myoglobin molecules in their blood, for efficient capture and transport of oxygen. When cetaceans dive, their heart rates slow by as much as 80 beats per minute, so their bodies use less oxygen than they would otherwise. (Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Cetaceans are found in all climates, including regions where sea water is near freezing. Small cetaceans can cope with cold temperatures because they have high metabolic rates. Also, their flippers and flukes have a countercurrent heat exchange system, wherein heat from arterial blood warms venous blood as it returns to the heart. Large cetaceans have a small surface area to volume ratio, so they lose little heat to the surrounding environment. Both small and large cetaceans are insulated by their thick blubber layer.

Reproduction

Cetaceans are difficult creatures to study, and for this reason, the behavior and biology of many species remains a mystery. Most species that have been studied exhibit polyandry, polygyny or polygynandry. The large testis size of some (such as bowhead whales, Balaena mysticetus) indicates sperm competition. Sometimes, as with northern right whales (Eubalaena glacialis), courtship behavior can be observed at the surface. In this species, multiple males congregate around a single female and jostle for position in a frenzy of activity. The female may mate with several males in succession, or even with two at the same time. (Nowak, 1999; Reeves, et al., 2002)

Most cetacean species have just one mating season per year. Females each give birth to a single calf every one to six years, after a 10 to 17 month gestation. Minke whales (Balaenoptera acutorostrata) have a postpartum estrus. Calving usually occurs in the warm season. Calves are born tail-first and must swim from the moment of birth. Female cetaceans nurse their calves for anywhere from six months (many mysticetes) to over two years (many odontocetes: 13-year-old sperm whales have been found with milk in their stomachs). Calves grow quickly (nursing blue whale calves gain 90 kg per day) but do not reach sexual maturity for at least two years, and mysticetes do not reach sexual maturity until they are at least 10 years old. Even after sexual maturity, cetaceans may have to wait several years before they are socially mature enough to breed. (Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Female cetaceans nurture their young inside their bodies for at least ten months, and then they provide them with milk extremely rich in protein and fat for at least six months. They are attentive mothers, helping their calves reach the surface to take their first breaths after they are born and keeping an eye out for predators. In some species, such as belugas (Delphinapterus leucas) mothers and calves form associations that last long after calves are weaned. Male parental care is rare, but males have been reported anecdotally to show interest in their offspring and bring them food. Calves are precocial, as they must be able to swim from birth. (baiji.org Foundation, 2005; Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Lifespan/Longevity

Because of the difficulties involved with tracking and studying cetaceans, lifespans are difficult to estimate. Available estimates suggest that most species live at least two decades, and some live much longer than that. One-hundred and sixteen-year-old fin whales (Balaenoptera physalus) have been reported from the wild and bowhead whales (Balaena mysticetus) may live up to 200 years. (Carey and Judge, 2002; George, et al., 1999)

Behavior

All cetaceans are completely natatorial. Swimming is accomplished by pumping the tail and flukes up and down and using the flippers for stability. Mysticetes can reach speeds of up to 26 km/hr when swimming; odontocetes can swim more than 30 km/hr. Many small odontocete species gain extra speed by riding waves created by wind or by boats. Cetaceans inhale before diving; they stay underwater anywhere from a few seconds to over an hour at a time. Some of the odontocetes, such as sperm whales (Physeter catodon), regularly reach depths of over 1,500 meters. (Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Some species of cetaceans (e.g., humpback whales, Megaptera novaeangliae) are known for their seasonal, long-distance migrations between temperate feeding grounds and tropical breeding grounds. Not all cetaceans make such extensive movements; some migrate on a smaller scale and others stay within one general area for their entire lives. Daily activity patterns of cetaceans are poorly known. Most activity is observed in the daytime, of course, this is when observations are easiest to make. Spinner dolphins (Stenella longirostris) are known to feed at night. (Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Communication and Perception

Cetaceans produce a variety of sounds. Baleen whales moan, grunt, chirp, whistle, and click to communicate; these sounds are made by the larynx. Male humpback whales (Megaptera novaeangliae) "sing" for up to 40 minutes at a time, presumably to attract females. Low-pitched moans produced by some baleen whales may be the loudest sounds produced by any animal; they carry underwater for hundreds of kilometers. Odontocetes communicate with whistles; these sounds are most likely produced by opening and closing nasal plugs. Odonotocetes also use clicks for echolocation, to navigate and to find food. They have a fatty organ called a melon on the forehead that focuses acoustic signals as they are emitted, and they receive sounds in the middle ear via the mandible. Odonocetes have a hearing range that greatly exceeds that of humans; they can perceive ultrasounds up to 120 kHz.

Though they rely mainly on sound to communicate, most cetaceans are able to see fairly well in both water and air. River dolphins (Inia, Lipotes, Platanista, Pontoporia) are the exception; there is little use for vision in the turbid waters where they reside, so their eyes are greatly reduced and some are nearly blind. Social odontocetes use touch extensively with pod-mates, which may be an important form of communication. (Nowak, 1999; Reeves, et al., 2002; Rice, 1984)

Economic Importance for Humans: Positive

For centuries, cetaceans have been hunted for their meat, blubber, and oil. The late 19th and 20th centuries saw a boom in commercial whaling to provide oil for lighting and heating and facilitated by newer and deadlier technologies for hunting whales. However, in 1986 a moratorium on commercial whaling was enacted by the International Whaling Commission, and today only Norway continues a commercial harvest. Harvesting for subsistence and scientific purposes continues, however, and several countries, including Iceland and Japan, continue to hunt whales under the name of "scientific" whaling. Currently cetaceans are important for entertainment and tourist industries: captive odontocetes are trained to perform tricks for large crowds of spectators, and whale-watching boats are popular attractions for tourists wishing to catch a glimpse of cetaceans in the wild. Whales are also important cultural and mythological icons for peoples around the world. (Nowak, 1999; Price, et al., 2005; Reeves, et al., 2002)

Contributors

Glossary

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

diurnal

active during the day, 2. lasting for one day.

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

echolocation

The process by which an animal locates itself with respect to other animals and objects by emitting sound waves and sensing the pattern of the reflected sound waves.

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

nomadic

generally wanders from place to place, usually within a well-defined range.

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

piscivore

an animal that mainly eats fish

planktivore

an animal that mainly eats plankton

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polyandrous

Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

polygynous

having more than one female as a mate at one time

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

sexual ornamentation

one of the sexes (usually males) has special physical structures used in courting the other sex or fighting the same sex. For example: antlers, elongated tails, special spurs.

social

associates with others of its species; forms social groups.

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

ultrasound

uses sound above the range of human hearing for either navigation or communication or both

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

young precocial

young are relatively well-developed when born

References

Boisserie, J., F. Lihoreau, M. Brunet. 2005. The position of Hippopotamidae within Cetartiodactyla. Proceedings of the National Academy of Sciences, 102: 1537-1541.

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This material is based upon work supported by the
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Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.